During their embryonic development, all chordates pass through a stage called the pharyngula [View] with these features:

a dorsal, tubular nerve cord ("1") running from anterior to posterior. At its anterior end, it becomes enlarged to form the brain.

a flexible, rodlike notochord ("2") that runs dorsal to the digestive tract and provides internal support. In vertebrate chordates, it is replaced by a vertebral column or backbone long before maturity.

pairs of gill pouches. These lateral outpocketings of the pharynx are matched on the exterior by paired grooves. In aquatic chordates, one or more pairs of gill pouches break through to the exterior grooves, forming gill slits ("3"). These provide an exit for water taken in through the mouth and passed over the gills.

a tail that extends behind the anus

There are three subdivisions of the chordates:

Cephalochordata

Tunicata

The cephalochordates and tunicates never develop a vertebral column. They are thus "invertebrates" and are discussed with the other invertebrates. [Link]

Craniata
The vast majority of chordates have a skull enclosing their brain, eyes, inner ear, etc.). All but one group of these (the hagfishes) also convert their notochord into a vertebral column or backbone thus qualifying as vertebrates.

Although hagfishes, never replace their notochord with a vertebral column, and thus might seem not to qualify as vertebrates, they share a number of other features with other vertebrates and certainly should be classified with them. Still uncertain is whether they represent the most primitive vertebrates or are simply degenerate vertebrates (probably the latter).

All the other members of the craniata convert their notochord into a vertebral column or "backbone" (even though in some it is made of cartilage not bone). They also differ from all other animals in having quadrupled their HOX gene cluster; that is, they have 4 different clusters of HOX genes (on 4 separate chromosomes). Perhaps this acquisition played a key role in the evolutionary diversity that so characterizes the vertebrates.

Lampreys and hagfishes are the only jawless vertebrates to survive today. They both have a round mouth and for this reason are often referred to as cyclostomes. They are the most primitive of the vertebrates. By "primitive", a biologist means that they are the least changed from the first vertebrates.

Besides lacking jaws,

They have no paired pectoral (shoulder) or pelvic (hip) fins.

Their notochord persists for life, never being completely replaced by a backbone even in the lampreys.

They have no scales.

The axons of their neurons are unmyelinated (like those of all invertebrates).

Lampreys have both an innate immune system and an adaptive immune system, but the latter is entirely different from that found in the jawed vertebrates.

The photo (courtesy of the Carolina Biological Supply Company) is of the West Coast lamprey. Note the gill slits and the absence of paired pectoral and pelvic fins.

Fossils of cartilaginous fishes become abundant in deposits dating to the Devonian period. They were very much like the sharks of today.
The group, which today is made up of some 1,188 species of

sharks

skates and

rays

gets its name from the fact that their skeleton is made of cartilage, not bone.

With their gills exposed to sea water, all marine fishes are faced with the problem of conserving body water in a strongly hypertonic environment. Sea water is about 3.5% salt, over 3 times that of vertebrate blood. The cartilaginous fishes solve the problem by maintaining such a high concentration of urea in their blood (2.5% — far higher than the ~0.02% of other vertebrates) that it is in osmotic balance with — that is, is isotonic to — sea water.

This ability develops late in embryology, so the eggs of these species cannot simply be released in the sea. Two solutions are used:

Enclose the egg in an impervious case filled with isotonic fluid before depositing it in the sea.

Retain the eggs and embryos within the mother's body until they are capable of coping with the marine environment.

Both these solutions require internal fertilization, and the cartilaginous fishes were the first vertebrates to develop this. The pelvic fins of the male are modified for depositing sperm in the reproductive tract of the female.

There are over 30,000 species (representing more than half of all living vertebrates).

They are an important part of the human diet in many areas of the world and, in affluent nations, support a large sports fishing industry.

Although the earliest bony fishes may have appeared late in the Silurian period, their fossils become abundant in freshwater deposits of the Devonian period. In addition to gills, these fishes had a pair of pouched outgrowths from the pharynx which served as lungs. They were inflated with air taken in through the mouth and may have provided a backup gas exchange organ when the water became too warm and stagnant to carry enough dissolved oxygen. Their kidneys were adapted for the hypotonic environment in which they lived. [Illustrated discussion]

These animals diversified through the remainder of the Devonian period (which is often called the "Age of Fishes"). Some migrated to the oceans. In this more stable environment, their lungs became transformed into a swim bladder with which they could alter buoyancy. Their kidneys became transformed as well adapting them to their new — hypertonic — surroundings. [Link to discussion]

two species of coelacanths. Coelacanths were long thought to have become extinct at the end of the Mesozoic era, some 70 million years ago. But in December 1938, a living coelacanth, Latimeria chalumnae, was pulled up from the depths of the ocean off the east coast of Africa. Since then, over 200 additional specimens have been caught.

several species of lungfish found in Africa, South America, and Australia.

The nostrils of bony fishes open only to the outside and are used for smelling. Some of the lobe-finned fishes developed internal openings to their nostrils. This made it possible to breath air with the mouth closed as modern lungfishes do.

Judging from present-day lungfishes, two other significant adaptations evolved in this group:

two atria and a partial septum in the ventricle of the heart (similar to the frog heart). This permitted a partial separation of oxygenated blood returning from the lung(s) and the deoxygenated blood returning from the rest of the body.

an enzyme system to convert ammonia into the less toxic urea. This mechanism is highly-developed in the African and South American lungfishes. While in the water, these fishes excrete their waste nitrogen as ammonia, just as most ray-finned fishes do. In time of drought, these animals burrow in the mud and switch to urea production.

These rare modern lobe-finned fishes are the sole survivors of once-flourishing groups that also gave rise to the tetrapods — the four-legged vertebrates. In the Devonian (perhaps as early as 395 million years ago), the paired fins of some sarcopterygians moved under the body and developed limbs (complete with digits). This enabled them to venture out on land. So once again, evolution was opportunistic giving rise to the first land vertebrates, the amphibians.

The figure shows the relationship between the bones of two tetrapod forelegs and the pectoral fin of a sarcopterygian.

With their bony limbs and lungs inherited from their lobe-finned ancestors, amphibians were so successful during the Carboniferous (Mississippian and Pennsylvanian periods) that these periods are known as the Age of Amphibians.

The Carboniferous was followed by the Permian, when the earth became colder and dryer. The fortunes of the amphibians began to decline until only three groups — totaling about 6500 species — remain today:

frogs and toads (Anura) (The one pictured is Rana pipiens, the leopard frog.)

salamanders and newts (Urodela)

caecilians (Apoda), which are rare, limbless, tropical animals.

As the name suggests, amphibians are only semiterrestrial:

Their skin is soft and moist so they are at risk of desiccation in dry surroundings.

Their eggs have no waterproof covering so

they must be laid in water (which makes them useful animals for studying embryonic development) where they are fertilized or

placed within the mother's body (some use a pouch in the skin, some use their mouth, some even use their stomach — which stops secreting acid and enzymes for the duration!) after external fertilization.

With the arrival of the cold, dry Permian, reptiles were well-adapted to survive because of their development of a shelled, yolk-filled egg which could be deposited on land without danger of drying out.

The photo (courtesy of the Carolina Biological Supply Company) shows an American chameleon emerging from its egg.

Other adaptations that enabled the reptiles to flourish for the next 220 million years were:

Beginning late in the Paleozoic era and exploding in the Triassic period, the reptiles underwent a remarkable adaptive radiation producing the diapsids.

This group developed the ability to convert their nitrogenous waste into uric acid. Uric acid is almost insoluble in water so its excretion involves little loss of water. (It is the whitish paste that pigeons leave on statues.) This modification largely freed the diapsids and their descendants from a dependence on drinking water; the water in their food is usually sufficient.

The only prototheria to survive until now are the monotremes: the duckbill platypus and three species of spiny anteaters (echidnas). These animals retain several traits of their therapsid ancestors including

a cloaca — the final segment of the digestive tract into which both the urinary and reproductive tracts empty (monotreme = single hole);

some 160 million years ago, it was not until the extinction of the dinosaurs at the end of the Cretaceous (~66 million years ago — Link) that they began the explosive adaptive radiation producing all the orders that we see today.

Their amniotic egg has no shell and develops for only a brief time within the reproductive tract of the mother. During this time they receive some nourishment from the yolk sac which grows into the wall of the uterus. They are born at such an early stage of development [View] that they must crawl into a pouch on the mother's abdomen, attach themselves to one of her milk-dispensing nipples, and complete development there.

Outside of Australia, the only marsupials found today are the opossums — one species in North America, some 69 species in South America. In Australia, however, where they were protected far longer from an influx of placental mammals, there is still an broad array of marsupials. [View, for example, the wombat.]

The placental mammals, with some 4,500 species living today, retain their shell-less amniotic egg within the mother. The egg contains little yolk, but the extraembryonic membranes form an

umbilical cord and

placenta

through which the young secure nourishment (and oxygen) from their mother until they are well-developed and ready to be born.

The table shows the orders of placental mammals and representatives of each.

The members of orders shown with the same color belong to a single clade; that is, they are more closely related to each other than to the mammals in any other orders.

These clades are based on the most recent analysis of DNA sequences of several genes — both nuclear and mitochondrial — and the building of phylogenetic trees from these data.

Tubulidentata

aardvarks

Macroscelidea

elephant shrews

Hyracoidea

hyraxes

Sirenia

manatee, dugong

Proboscidea

elephants

Cingulata

armadillos

Pilosa

anteaters, sloths

Dermoptera

flying lemurs

Primates

lemurs, New World monkeys, Old World monkeys, great apes (hominoids), humans

Scandentia

tree shrews

Rodentia

rats, mice, squirrels, beaver

Lagomorpha

rabbits, hares

Insectivora

shrews, moles

Chiroptera

bats

Pholidota

scaly anteaters

Carnivora

dogs, cats, lion, skunk, walrus, sea lion

Perissodactyla(odd-toed ungulates)

horse, zebra, rhinoceros

Cetartiodactyla(even-toed ungulates and cetaceans)

cow, sheep, piggiraffe, hippopotamus and whales, dolphins, porpoises

Afrotheria

The 5 orders shown in yellow are found in Africa. They appear to be the most primitive of the placental mammals.

Xenarthra

The 2 orders shown in red (armadillos, sloths, giant anteaters) probably arose in the part of Gondwana that today is South America.

Euarchontoglires

The hugely-successful glires (rodents and rabbits) form a clade (light blue) that is so closely related to the clade (dark blue) that includes the tree shrews, the flying lemurs, and all our primate relatives, that they are grouped together. [Link to page on primate evolution.]

Laurasiatheria

These 6 orders, shown in green, have been assigned to the Laurasiatheria because they are first found in the supercontinent of Laurasia. [ map and discussion ]

The even-toed ungulates and the cetaceans turn out to be so closely related that they are placed in a single order the Cetartiodactyla. In fact, hippos appear to be more closely related to the cetaceans than they are to the other even-toed ungulates.